The present invention relates to focussing means for a bar code reader, comprising a lens system and drive means, the lens system comprising at least one lens or set of lenses moveable in the direction of its optical axis, the drive means comprising at least one coil and a magnetic element. The invention further relates to a bar code reader comprising such focussing means.
A bar code reader comprising such focussing means is disclosed in the German patent application DE-A-3,736,288.
Bar code readers generally comprise focussing means, such as a lens system, for focussing light scattered by a bar code onto a sensor or an array of sensors. Most known focussing means have the disadvantage that the focussing is highly dependent on the distance of the bar code relative to the reader. If the distance between the bar code and the reader is changed, the image of the bar code on the sensor is out of focus, resulting in a decreased likelihood that the code is read correctly. If the distance is changed even further, the reader may not be able to read the code at all.
Several arrangements have been proposed for providing focussing means having an extended working range, i.e. an extended range of distances in which the image of the bar code on the sensor (or an array of sensor elements) is substantially in focus. One such arrangement contains holographic elements positioned on a rotating wheel, each element having a different focal length. The variation in focal length is for this arrangement necessarily discontinuous.
The focussing means disclosed in the above-mentioned German patent application comprise drive means for varying the position of a moveable lens. These known drive means are constituted by a fixed permanent magnet and a moveable coil. The moveable lens is mounted in the moveable coil, while the coil is loosely and only partially inserted in the substantially annular permanent magnet. The moveable coil is connected to a control unit by means of coils leads.
The bar code reader disclosed in the said German patent application is designed for reading very small bar codes which are applied on semiconductor surfaces by means of dots having a diameter of approximately 15 .mu.m. Such semiconductor surfaces are generally precisely positioned relative to the bar code reader. The range of focussing distances of this known device is necessarily very small. The known focussing means therefore only allow a movement of the lens of typically .+-.1 mm. Such a small movement of the lens is however insufficient for most applications where e.g. bar codes applied on consumer articles have to be read. A typical point of sale bar code reader has to be able to correctly read bar codes at distances varying from a few to several dozens of centimeters. The above-mentioned known bar code reader does not allow such a reading range. Furthermore, the known bar code reader cannot easily be adapted to allow an expanded movement of the lens. The known structure, in which the relatively small movement is primarily limited by electromagnetic forces, is not suited for guiding the moveable part over a relatively large range of travel.